Sealing anodized aluminum



United States Patent 3,098,018 SEALING ANQDIZED ALUMINUM Gerald H. Kissin and Richard L. Smith, Spokane, Wash,

assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware No Drawing. Filed July 13, 1961, Ser. No. 123,655 9 Claims. (Cl. 204-35) The present invention relates to methods of, and means for sealing the oxide coating that is artificially formed on the surfaces of al-uminous structures, and more particularly, relates to the sealing of such oxide coatings as are formed by the use of an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and a metal sulfate or sulfuric In general, oxide coatings are usually produced by placing the aluminum part into a suitable electrolyte and passing an electric current through said electrolyte with the aluminum part serving as the anode. The resultant disintegration of the electrolyte releases oxygen which combines chemically with the aluminum and its alloying constituents to form a hard oxide layer on the surface of the aluminum part-a process known as anodizing. The oxide layer is usually subjected to a treatment which is known as sealing in order to prevent loss of color in the event that it is colored, and to render it generally more stable and resistant to corrosion.

This sealing may be accomplished in a variety of different ways. One form of sea-ling the coatings, particularly if they have been colored by an organic dyeing process, has been to expose the anodized aluminum parts to solutions containing substantial amounts of hydrolyzable metal salts, especially nickel acetate and/ or cobalt acetate. In certain instances this method produces an unsightly residue known as smut which can be observed upon the finished surfaces or appears after light rubbing. This residue impairs the appearance of the part and may have to be removed by suitable means, e.g., polishing or bufiing operations, which are cumbersome and costly and increase the cost of the finished article.

Attempts have been made to prevent the formation of smut on sealed, dyed aluminum surfaces by incorporating into the sealing solution substantial amounts of a metal acetate, a ligno-sulfionate and a buifering agent, such as boric acid. However, this method involves certain disadvantages. First of all, the cost of the treating bath is rather high due to the cost of its ingredients and the relatively large quantities thereof used. Secondly, the treat ing bath tends to be unstable and produces undesirable side effects after relatively short periods of use in that the oxide layers exposed thereto develop an unsightly brownish cast. As a result, the useful life of such treating baths in relatively short.

During an investigation of the anodized oxide coatings which are disclosed in a co-pending patent application entitled Method and Composition for Treating Metal and Article Produced Thereby, by Deal et al., S.N. 857,562, filed December 7, 1959, now Patent No. 3,031,387, which is assigned to the same assignee as the subject application, it was determined that the prior art sealing baths were not completely satisfactory for these coatings because of the formation of an unsightly smut, in addition to the other above-mentioned disadvantages.

in general, these coatings can be formed by using an electrolyte including sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid as set forth below and in the abovementioned Deal et al. application. The electrolyte consists essentially of an aqueous solution of from about to 50% by weight of su-lfosalicylic acid and at least one substance selected from the group consisting of metal sulice fates and sulfuric acid in an amount equivalent to not more than 15% by weight sulfuric acid. Of the recited gnoup it is presently preferred to use sulfuric acid. Ferric sulfate is a preferred metal sulfate. Even a small amount of metal sulfate or sulfiuric acid is beneficial to the electrolyte embodying the principles of this invention, e.g., an amount of metal sulfate or sulfuric acid equivalent to 0.1% by weight of sulfuric acid is beneficial. The preferred ranges are from about 7 to 15 by weight sulfosalicylic acid and an amount of metal sulfate or sulfuric acid equivalent to from about 0.1 to 4% by weight sulfuric acid.

It has been found that these oxidized aluminous surfaces may effectively be sealed at a substantial saving in cost and without the formation of smut on, or discoloration of, the finished product by immersion into a sealing bath containing relatively small and well-defined amounts of a metal ion supplier, such as nickel acetate and a substance such as a ligno-sulfonate. In fact, the invention utilizes less than 4 grams of total additives per liter of bath, with preferably about one to about five times as much li-gno-sulfona-te as metal ion supplier. The bath should also be maintained at a pH value of between 4 and 6.

In the following examples, commercial aluminum alloys were used having compositions as set forth in Table I. Of course, these are merely examples, as other alloys can be used.

TABLE I Per- Per- Per- Per- Per- Per- Per- Per- Alloy cent cent cent cent cent cent cent cent Fe Ou Mn Mg Zn Tl Or In Table I the balance of the rnaterials in each case is aluminum.

Alloys 5052 and 606l-T6 were in the form of sheets having a thickness of 0.040 inch, while alloys 6063-T5 and 6351-T5 were in the form of extrusions. These ailloys were electrolytically anodized as follows:

(1) Cleaned in a suitable inhibited alkaline cleaner at a concentration of 65 grams per liter for 5 minutes at F. An example of one suitable alkaline cleaner is one composed of 40 grams per liter of sodium carbonate, 20 grams per liter of sodium phosphate, 5 grams per liter of sodium metasilicate, balance water.

(2) Rinsed in cold Water.

(3) Etched for 5 minutes at 160 F. in 5% by weight caustic soda solution.

(4) Rinsed in cold water.

(5) Dipped for 2 minutes in 30% by weight nitric acid at room temperature.

(6) Rinsed in cold water.

(7) Anodized at 24 amperes per sq. ft. at temperature of 25:1 C. in a bath of the following composition:

S-sulfosalicylic acid 95.9 Sulfuric acid 6.70

For 6061 and 6351 alloys, anodizing was continued at In step 10 above, the sealing baths used had the composition, and were operated under conditions, as follows:

Example 1 Nickel acetate 'grams/liter 0.5 Sodium ligno-sulfonate do 1.0 Balance Water Temperature F 210 pH 4.9 Time minutes 20 Also, temperatures of 203 F., with time of 15 minutes and a pH of 5.0 were used.

With all the alloys mentioned in Table I, the anodized surfaces of the samples were completely sealed, as determined by their resistance to staining by dyes or inks. No smut formation could be observed on the surface of the finished article, and no brownish discoloration. The bath was continuously used for the treatment of anodized aluminous structures for a period of two days and remained fully effective. Also, its cost was much less than that of the above-described prior art conventional sealing baths employing nickel acetate, cobalt acetate, calcium ligno-sulfonate, and boric acid.

This bath also gave satisfactory results with the alloys of Table I.

Example 4 Sodium ligno-sulfonate grams/liter 5 Balance Water This bath gave poor results.

Example 5 Nickel acetate grams/liter 1.0 Sodium ligno-sulfonate do- 5 .0 Balance Water This bath gave poor results.

These and other tests have shown that the contents of nickel acetate of the bath can preferably vary from 0.3 to 0.6 gram/liter, but should never exceed about 0.8 gram/liter, otherwise smut formation can occur. Also, it should never \drop below 0.02 gram/liter to retain its effectiveness. The amount of ligno-sulfonate in the bath can preferably vary from about 0.5 to 2.0 grams/liter, but should at no time exceed 3.0 grams/liter, as otherwise the finished product may develop a brownish discoloration. It should never drop below 0.1 gram/liter to remain effective in preventing smut formation.

For the described results to be obtained with the small quantities of nickel acetate and ligno-sulfonate salt specified above, it is essential to keep the pH value of the solution between 4 and 6, preferably at or close to 5. An auxiliary buffer, such as boric acid, is not necessary.

For best results the temperature of the bath should be between F. and 212 F. and the oxidized aluminous structures should be exposed to the bath for pcriods ranging from 10 to 30 minutes.

In practice the additives of the bath of the invention may be shipped and stored as a dry mixture consisting of nickel acetate and a ligno-sulfonate in ratios ranging from about 1:1 to 1:5; and whenever needed, this mixture will be dissolved in water in such quantities as will yield a solution containing less than 4 grams of the mixture per liter of water.

Of course, substances other than ligno-sulfonate may be used in this invention if they function in an equivalent manner and therefore the invention is not to be construed as being limited in that respect. As used herein and in the claims, the term aluminous means high purity aluminum, commercial pure aluminum and aluminum based alloys containing at least 50% aluminum, by weight.

While the present invention has been shown in a few forms only, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope thereof as set forth in the appended claims.

What is claimed is:

l. A bath for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said bath consisting of an aqueous solution of from 0.02 to 0.8 gram/liter of nickel acetate and from 0.1 to 3.0 grams/liter of a l-igno-sulfonate and having a pH value of between 4 and 6.

2. A bath for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said bath consisting of an aqueous solution of from 0.3 to 0.6 gram/liter of nickel acetate and from 0.5 to 2.0 grams/liter of a ligno-sulfonate and having a pH value of about 5.

3. A bath for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said bath consisting of an aqueous solution of from 0.3 to 0.6 gram/liter of nickel acetate and from 0.5 to 2.0 grams/liter of a ligno-sulfonate having a pH value of about 5, said bath being maintained at a temperature between about 185 P. and 212 F. during operation.

4. A bath for sealing oxidized aluminous surfaces produced by the method or anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said bath consisting of an aqueous solution of about 0.5 gram/ liter of nickel acetate and about 1.0 gram/liter of sodium ligno-sulfonate and having a pH value of about 5.

5. A method for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said method comprising immersing said structure in a bath consisting of an aqueous solution of from 0.02 to 0.8 gram/liter of nickel acetate and from 0.1 to 3.0 grams/liter of a ligno-sulfonate and maintaining said bath at a pH value of between 4 and 6.

6. A method for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfos-alicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said method comprising immersing said structure in a bath consisting of an aqueous solution of from 0.3 to 0.6 gram/liter of nickel acetate and from 0.5 to 2.0 grams/liter of a ligno-sulfonate, and maintaining said bath at a pH value of about 5.

7. A method for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essential-1y of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said method comprising immersing said structure in a bath consisting of an aqueous solution of from 0.3 to 0.6 gram/liter of nickel acetate and from 0.5 to 2.0 grams/liter of sodium ligno-sulfonate for a period of from about 10 to 30 minutes, and maintaining said bath at a pH value of about 5, and at a temperature of between =about 185 F. and 212 F.

8. A method for sealing oxidized aluminous surfaces produced by the method of anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said method comprising immersing said structure in a bath consisting of an aqueous solution of about 0.5 gram/liter of nickel acetate and about 1.0 gram/liter of sodium ligno-sulfon-ate for a period of about 20 minutes, and maintaining said bath at a pH value of about 5, and at a temperature of about 210 F.

9. A mixture for the preparation of a bath for sealing oxidized aluminous surfaces produced by the method of 'anodic oxidation using an electrolyte consisting essentially of an aqueous solution of sulfosalicylic acid and at least one substance selected from the group consisting of metal sulfates and sulfuric acid, said bath being prepared by the addition of said mixture to Water and said mixture consisting of from 0.02 to 0.8 gram/ liter of nickel acetate and from 0.1 to 3.0 grams/liter of a ligno-sulfonate.

References Cited in the file of this patent UNITED STATES PATENTS 2,008,733 Tosterud July 23, 1935 2,888,388 Stiller May 26, 1959 3,031,387 Deal et a1 Apr. 24, 1962 

1. A BATH FOR SEALING OXIDIZED ALUMINOUS SURFACES PRODUCED BY THE METHOD OF ANODIC OXIDATION USING AN ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS SULFOSALICYLIC ACID AND AT LEAST ONE SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF METAL SULFATES AND SULFURIC ACID, SAID BATH CONSISTING OF AN AQUEOUS SOLUTION OF FROM
 0. 02 TO
 0. 8 GRAM/LITER OF NICKEL ACETATE AND FROM
 0. 1 TO
 3. 0 GRAMS/LITER OF A LIGNO-SULFONATE AND HAVING A PH VALUE OF BETWEEN 4 AND
 6. 